1. Field of the Invention
This invention relates to a method of recovery of heavy, viscous oil from subterranean oil-containing formations. More particularly, this invention relates to an improved tertiary method of recovering heavy oil, wherein the oil is pretreated with a solution which increases its mobility and thereafter a drive fluid is used to displace and recover the oil from the formation.
2. Background of the Invention
There are vast reserves of heavy oil contained in subterranean formations whose natural characteristics, such as high viscosity, low API gravity, and high molecular weight render recovery thereof by primary oil recovery techniques commercially unfeasible due to the lack of natural reservoir energy. In addition, these reserves have relatively low permeability although they may have high porosity.
Attempts have been made in the past to employ known oil displacement procedures for tertiary recovery of the oil by injecting fluid through the formation between injection and production wells. Such attempts include waterflooding, miscible flooding, chemical or micellar flooding, polymer flooding, thermal recovery by hot fluid injection, thermal recovery by in-situ combustion and solvent flooding. Such techniques improve the mobility of the oil, thereby enhancing the recovery of heavy oil.
Among these recovery techniques, the miscible flooding process using carbon dioxide (CO.sub.2) has often been used for enhanced oil recovery. The CO.sub.2 miscible flooding process is often applicable to high gravity crude oils, e.g., 25 API, and involves high operating pressures (starting at about 1500 psi). These particular conditions under which the process is practiced produce true miscibility of oil or hydrocarbons with CO.sub.2 thus forming a single phase, which can be recovered relatively readily from underground formations.
Attempts were made in the past to employ carbon dioxide for the recovery of heavy crude oils. Heavy crude oils are classified as those having a low API gravity and/or high viscosity which are difficult to produce by ordinary methods of primary and secondary oil recovery. They also contain very small amounts of low boiling components. At present, there is an increased interest in the application of carbon dioxide gas for reducing the viscosity of heavy oils thus causing a decrease in the effect of viscous forces associated with the in-situ recovery of heavy oils. However, preliminary review of the published literature suggests that the benefit of carbon dioxide in the miscible flooding processes may be less promising than expected. Even as a dissolved gas, the application of CO.sub.2 in heavy oil does not look promising because: (i) the solubility of CO.sub.2 in heavy oils having a low API gravity is lower than that in light oils, (ii) for pressures corresponding to reservoir depths of 2000 ft. or below, the solubility of CO.sub.2 in H.sub.2 O is comparable to that in heavy crude oil which competitively reduces the effective dissolution of the CO.sub.2 gas in heavy crude oils, (iii) CO.sub.2 compression costs for deep reservoirs are excessive, and (iv) the solubility of CO.sub.2 markedly decreases above 80.degree. C.
One of the most widely used secondary supplemental oil recovery techniques is waterflooding which involves the injection of water into an oil-containing formation. As the water moves through the formation, it acts to displace oil to a production system composed of one or more wells through which the oil is recovered. It has also been proposed to add surfactants to the injected water to lower the oil-water interfacial tension and/or alter the wettability characteristics of the formation rock to enhance recovery of the oil. Various surfactant waterflooding techniques are disclosed in U.S. Pat. No. 3,469,630 to Hurd et al. and U.S. Pat. No. 3,977,470 to Chang.
Another waterflooding technique is taught in U.S. Pat. No. 3,757,861 to Routson which discloses the introduction into the formation of an aqueous solution of peroxide, typically hydrogen peroxide, and thereafter or simultaneously the introduction of an aqueous solution of alkali metal hydroxides or carbonates or ammonium hydroxide.
Ralfsnider et al, U.S. Pat. No. 3,532,165 disclose the injection into an underground formation of a concentrated solution of an inorganic acid, e.g., hydrochloric or sulfuric acid, followed by the injection of a stoichiometric excess of a saturated solution of an inorganic salt, such as sodium carbonate. Alternatively, an aqueous solution of a sodium carbonate or bicarbonate is injected into the formation, followed by the injection of sodium sulfite or bisulfite. Both methods generate in-situ carbon dioxide.
However, there is still a need in the art of recovering heavy oils (gravity of not more than 20.degree. API) and medium gravity oils (gravity of 20.degree.-35.degree. API) for improved recovery techniques which produce higher amounts of such oils than previously-known methods.